Add defined_names

[danvk]

Add DefinedName and DefinedNameList stubs. These are based on the auto-generated stubs in VS Code, which I believe are generated by pylance. I confirmed that these stub files fixed the type errors I saw in my own code when I installed the openpyxl-stubs.

See:

• https://openpyxl.readthedocs.io/en/stable/api/openpyxl.workbook.workbook.html
• https://openpyxl.readthedocs.io/en/stable/defined_names.html

Closes #23

[MartinThoma]

0.1.25 was just released with this fix